The liquid crystal display (hereinafter may also be referred to as the LCD) is being used extensively as a main device of multimedia-related devices in recent years, but with the expansion of its use, there are demands for weight reduction, thickness reduction, reduction of power consumption, provision of high luminance and cost reduction. Among the LCDS, a high-definition display device is required for displays for personal computers, vehicle-mounting displays, TV monitors and the like. Meanwhile, since a liquid crystal display element itself does not emit light, a transmission type liquid crystal display element using a back light having a fluorescent lamp as a light source is used for the above-described usage. And, for devices using a reflection type liquid crystal display element, a front light is used as a light source for emitting light from the front.
With the trend toward the weight reduction, thickness reduction, provision of high luminance and reduction of power consumption of the LCD, the fluorescent lamp for the back light is also under progress for provision of a narrow tube and a small wall thickness. The provision of a narrow tube and a small wall thickness of the fluorescent lamp degrades a mechanical strength, and the improvement of a luminous efficiency tends to increase the heating value of the lamp. Therefore, a glass having a higher mechanical strength and heat resistance is being required.
Under the circumstances described above, in order to provide a higher strength and heat resistance compared to a conventionally used lead-soda soft glass, a fluorescent lamp using a borosilicate hard glass has been developed and put on the market. A kovar alloy or tungsten has been used for the enclosed wires of electrodes, and a low expansion borosilicate glass sealable airtight with such a metal has been developed. The “kovar” used here is a trademark (trade name) indicating an Fe—Ni—Co alloy of Westinghouse Ele. Corp., and it is used in a sense including the equivalent products of other companies, such as a KOV (brand name) produced by Toshiba Corporation.
The low expansion borosilicate glass is diverted from a glass generally used for the conventional xenon flash lamps. In a case where the glass is used for the xenon flash lamps, it is designed such that a certain level of ultraviolet rays is allowed to pass through it so as to resist the flash of light of the lamp. But, in a case where the glass is used for the fluorescent lamps, it is necessary to consider measures to prevent leakage of ultraviolet rays and discoloration of the glass by radiation of ultraviolet rays produced within the lamp, so-called ultraviolet solarization, and a glass to which a small amount of components for improving such properties is added is being used.
The glass disclosed in Patent Reference 1 or Patent Reference 2 is a typical example of a glass for the above-described usage, and it has a composition with the resistance to ultraviolet solarization of the glass improved by containing any of TiO2, PbO and Sb2O3 with a borosilicate glass used as a base. And, the glass disclosed in Patent Reference 3 or Patent Reference 4 has a composition with the ultraviolet transmittance of 253.7 nm, which is a resonance line of mercury, suppressed to a low level by further adding Fe2O3 and CeO2.
In mass production, the glass tube is produced by an up drawing method, a Vello process, a Danner method and the like, but since the glass tube used for the back light is a thin tube and required to have high dimensional accuracy, the Danner method is optimum.
Patent Reference 1: JP-A 9-110467 (KOKAI)
Patent Reference 2: JP-A 2002-187734 (KOKAI)
Patent Reference 3: JP-A 2002-293571 (KOKAI)
Patent Reference 4: JP-A 2004-91308 (KOKAI)